by Trevor Hughes, USATODAY

by Trevor Hughes, USATODAY

Because there's no quick chemical or breath test for marijuana available to most police officers, standard voluntary roadside tests have taken on new importance as states increasingly legalize marijuana use.

Voluntary roadside tests check a driver's ability to do several things at once, known as "divided attention."

The tests administered by police officers are designed to check your ability to follow directions and track the passage of time, along with your ability to move your body properly. Marijuana and alcohol can affect your ability to follow directions and move carefully.

The tests aren't intended to trick anyone and can easily be completed by a sober driver. In many cases, the police officer looks for subtle signs of impairment such as how much you blink or whether your eyes bounce or cross when following the tip of a pen.